Phase locked loop circuits are utilized in a myriad of applications for providing an output signal having a frequency that tracks the frequency of an input reference signal. The phase lock loop circuit typically includes a phase detector which is responsive to an input reference signal and a feedback signal for providing output signals to a charge pump circuit. The charge pump circuit subsequently provides an output voltage to a voltage controlled oscillator (VCO) thereby determining the frequency of the VCO. Optionally, the output of the VCO undergoes a frequency division via a divider network wherein the output of the divider network is the feedback signal to the phase detector.
Briefly, if the frequency of the reference signal is greater than the frequency of the feedback signal, the phase detector will provide an output signal to the charge pump circuit which has the effect of providing a voltage to the VCO to increase the output frequency of VCO. The output signal of the VCO is then fed back, via the divider network, and is again compared with the input reference signal. This process is continued until the input reference signal is substantially equal to the feedback signal.
However, the VCO is typically able to run faster than other circuitry in the phase locked loop. Further, if the VCO begins to outrun the feedback circuitry, the feedback signal will be lost causing the phase detector to further increase the speed of the VCO. Unfortunately, this condition results in the VCO running at its maximum frequency thereby inhibiting the phase lock loop from recovering its lock to the input reference signal.
Hence, there exists a need to provide a lock recovery circuit that detects when lock has been lost and to restore the phase lock loop to a phase locked condition.